1. Field of the Invention
The present invention relates generally to AC motor drive control systems, and relates more particularly to improving performance of AC motor drive control systems.
2. Description of Related Art
Sensorless AC drives provide a number of advantages over motor drive systems that include position feedback devices such as encoders or resolvers. Operating AC drives in a sensorless control scheme can reduce system costs and simplify implementation, and provide a number of other advantages in typical environments in which AC motor drives are employed.
Sensorless AC drive control schemes often benefit from a voltage feedback derived from the power signals supplied to the motor. A closed loop voltage feedback can improve the control system responsiveness, and in particular when used with the current feedback typically employed in a sensorless AC motor drive. One advantage of voltage feedback is improved drive performance especially at low speed operation. A number of non -linearities and other system variables can be largely suppressed by providing a closed loop voltage control. For example, inverter dead time, conduction losses and switching losses, which are typical non-linear variables in the motor drive system can be compensated with a high bandwidth voltage regulating loop. Such a voltage regulating loop may operate at 1500 rad/sec, for example. Sophisticated motor drive control systems also use on line motor parameter estimation algorithms that use motor voltages as input signals. Accordingly, using voltage feedback for AC drive control systems permits a number of high performance control opportunities in the implementation of an AC drive system.
One of the difficulties with voltage feedback sensing is a DC offset typically found in the feedback circuitry. A DC offset in the feedback circuitry can cause unwanted drive torque oscillations, and can even lead to unstable drive operation. One technique to solve the problem of a DC offset is to nullify the sensor feedbacks at initialization, prior to operating the drive system. However, once the motor drive is initialized and running, it is difficult to continue to nullify the DC offset due to the presence of high amplitude AC signals. For the sake of ease of implementation, offset compensation is typically frozen until the motor drive is no longer running, at which point a DC offset compensation can again be performed. DC offset voltage may vary significantly during long duration runs of a motor drive, and significant unwanted torque ripple can eventually develop as the DC offset voltage continues to build. It would be desirable to eliminate the DC offset while the motor drive system is on line and running.